Problem: Which of the following numbers is a factor of 156? ${8,9,10,12,14}$
By definition, a factor of a number will divide evenly into that number. We can start by dividing $156$ by each of our answer choices. $156 \div 8 = 19\text{ R }4$ $156 \div 9 = 17\text{ R }3$ $156 \div 10 = 15\text{ R }6$ $156 \div 12 = 13$ $156 \div 14 = 11\text{ R }2$ The only answer choice that divides into $156$ with no remainder is $12$ $ 13$ $12$ $156$ We can check our answer by looking at the prime factorization of both numbers. Notice that the prime factors of $12$ are contained within the prime factors of $156$ $156 = 2\times2\times3\times13 12 = 2\times2\times3$ Therefore the only factor of $156$ out of our choices is $12$. We can say that $156$ is divisible by $12$.